Compared to combustion using air, oxy-combustion allows an energy saving at least on the basis that the energy of the combustion gases is not partially absorbed by the nitrogen in the air. In traditional furnaces, even if part of the energy entrained with nitrogen is recovered, the fumes containing nitrogen that are ultimately discharged still entrain a significant proportion thereof.
Reducing the energy consumption per production unit in question has the additional advantage of limiting carbon dioxide emissions and therefore enabling regulatory requirements in this field to be met.
The presence of nitrogen is, moreover, a source of the formation of so-called NOx oxides, the emission of which is forbidden in practice because of the damage associated with the presence of these compounds in the atmosphere.
In practice, users endeavour to operate furnaces that work with air in conditions that result in the most limited amount of emissions possible. In the case of glassmaking furnaces, these practices are not sufficient to meet the very strict standards in force, and it is necessary to conduct a costly decontamination of fumes using catalysts.
In spite of the advantages outlined above, the use of oxy-combustion, in particular in large glassmaking furnaces, is yet to be developed. There are various reasons for this. One drawback lies in the cost of oxygen. This is compensated, in principle, by economies carried out elsewhere. Nevertheless, it is to be taken into consideration in that there are additional investments in specific equipment for the use of oxygen.
The economic evaluation of using oxy-combustion is only positive when the conditions of use are optimised. With respect to energy efficiency, this requires the use of oxygen at elevated temperature, i.e. a temperature of several hundreds of degrees.
Secondly, it is also expedient to preheat the fuel and also the batch materials to some extent.
The preheating of oxygen and its use are delicate operations. The difficulties that arise result in particular from the extremely corrosive nature of hot oxygen for materials used in contact with it, in particular metal alloys.
While the inventors in the unpublished prior application (European Patent Application No. 07 107 942 filed on 10 May 2007) proposed heat exchangers that enable the oxygen to be heated in a satisfactory manner, the use of this hot oxygen also poses specific problems in the operation of furnaces, and in particular in glassmaking furnaces.
It is known that large glass melting furnaces, i.e. furnaces having production capacities of several hundreds of tonnes per day, are constructed to operate without interruption for periods of more than ten years. This service life is determined mainly by the wear of the refractory materials forming the walls thereof.
During these very long periods the other elements of the furnace, and in particular the burners, must maintain all their initial performance levels. There are two main reasons that are likely to change their characteristics. On the one hand, the circulation of hot gases, in particular oxygen, causes wear of the injection elements. On the other hand, the fuels subjected to very high temperatures in the conduits close to the combustion zone can cause deposits that inhibit the flow of these fuels. This latter effect is noticeable in particular in the use of heavy fuels when these are exposed to temperatures exceeding 180° C., for example. However, the breakdown of gaseous fuels can also occur to a lesser degree, even if this only occurs to a significant extent at temperatures exceeding the 600s° C.
The injection means for the fuels must be maintained regularly throughout the service life of the furnace, whatever fuels are selected.
Maintenance operations must be conducted without interrupting the operation of the furnace. One problem is to design burners and their associated supply means in such a manner that these operations can be conducted in relatively convenient and rapid conditions. The inventors endeavour to propose means and ways of implementing these, so that this question can be answered.
The inventors have firstly noted that transporting hot oxygen requires the use of specific means. The highly corrosive nature of hot oxygen means that its circulation must be limited to the shortest possible routes. It is also necessary to avoid the impact of the flow of oxygen in conduits having curves that are too severe. It is also desirable to avoid any surface irregularities on the route of this gas. In particular, joints are preferably formed by welding, with these welds having a polished surface.
These requirements, as well as others, in the management of hot oxygen mean that the means used are substantially fixed. These means are therefore not suitable for the maintenance operations discussed above.